FR2873718A1 - PROCESS FOR FORMING A CONVERSION LAYER ON AN ALUMINUM ALLOY PRODUCT FOR BREATHING - Google Patents

Abstract

The subject of the invention is a process for forming a conversion layer on the surface of an aluminum alloy product to be assembled by soldering without flux comprising treatment with a solution containing ions. K + and F-and one or more acid (s) in an amount such that the pH of the solution is less than 3.The method according to the invention allows brazing without effective flow under industrial conditions, in particular for the heat exchangers used in the automobile.

Description

Process for forming a conversion layer on an alloy product

  BACKGROUND OF THE INVENTION The invention relates to a method of forming a conversion layer on the surface of an aluminum product, in particular a sheet or strip or a spun product such as a tube. , intended to be assembled by brazing without flux. These aluminum products are used in particular in the manufacture of heat exchangers, in particular those used for engine cooling and air conditioning of passenger compartments of motor vehicles.

  State of the art The brazing process currently the most used for the manufacture of heat exchangers is soldering in a controlled atmosphere with a non-corrosive flow, said Nocolok brazing. This process gradually replaces the vacuum brazing used previously but which required expensive installations both in initial investment and maintenance.

  The main disadvantage of the Nocolok process lies in the need to deposit on the metal, before soldering, a compound called flux and consisting of potassium fluoromelates. The purpose of this is to destroy the surface oxide layer on the aluminum alloys, a little before the beginning of melting of the veneer. Thus, the liquid metal will not be retained by this film and may appropriately wet the surfaces to be assembled and form sufficiently large and resistant joints. The use of this product, which is in the form of a powder, by automotive equipment manufacturers poses a number of problems: it causes in particular a relatively large dispersion of dust in the workshops.

  Many publications or patent applications offer solutions to overcome this problem. They can be classified into two broad categories: The first category concerns soldering processes without flux. Processes using products coated with a nickel deposit (see, for example, WO 02/07928 by Corus Aluminum Walzprodukte) and the addition of sodium in the plating alloy described in the WO application may be mentioned in this field. 01/98019 of Kaiser Aluminum & Chemical Corporation.

  The second category concerns so-called pre-fluxed strips, that is to say strips having a surface layer containing the soldering flux. In US Patent No. 6,120,848 to Ford Motor Company, the flux is mechanically inlaid into the strip during rolling. In the applications WO 00/64626 of Corus Aluminum, WO 02/087814 - WO 03/037559 of Norsk Hydro, or WO 03/089176 of Alcoa Inc. and American Inks and Coatings Corporation, the flux is incorporated in a binder and everything is deposited on the aluminum strip. In US Pat. No. 6,344,237 to Alcoa Inc. the flux is embedded in the metal by high speed spraying.

  Ford Global Technologies patent EP 1067213 discloses the treatment of an aluminum strip in an aqueous solution of 2 to 25% by weight of KF at a temperature between 32 and 100 C for at least 5 seconds. This operation would create on the surface of the metal a conversion layer which contains either substantially K3A1F6 or mainly K3A1F6 and in addition a small proportion of KA1F4.

  The Applicant has found that the implementation of the method described in this patent did not allow to obtain a brazing without satisfactory flow.

  OBJECT OF THE INVENTION The subject of the invention is a process for forming a conversion layer on the surface of an aluminum alloy product intended to be assembled by soldering without flux comprising the treatment with the aid of a solution containing K + and F "ions and one or more acid (s) in an amount such that the pH of the solution is less than 3.

  The acid is preferably sulfuric acid at a concentration of between 10 and 40 g / l. The treatment is preferably at a temperature between 30 and 100 C.

Description of figures

  Figures 1a and 1b show, respectively in top view and side view, the V-shaped specimens used in the examples to evaluate the solderability.

Description of the invention

  The invention consists in modifying the treatment solution described in patent EP 1067213 to obtain a deposit for brazing without additional addition of flux. This modification consists of an acidification of the bath, without using hydrofluoric acid (which is mentioned in the application EP 0140267 of Toyota Chuo) which poses too great toxicity problems.

  The invention consists in adding to the KF solution described in the aforementioned patent sulfuric acid at a level of 10 to 40 g / l. It would also be possible to use nitric acid HNO3 at a concentration of 20 to 50% by volume, but this solution is not preferred because of the possible emission of nitrous vapors. It is also possible to use hydrochloric acid HCl (typically at a concentration of 1 to 50 g / l), one or more organic acids, or any combination of these mineral or organic acids which makes it possible to obtain a pH of less than 3, to place in the zone of dissolution of aluminum. On the other hand, it is preferable to avoid phosphoric acid, for reasons of compatibility with the brazing process for which phosphorus has a deleterious effect.

  The other treatment conditions recommended in the patent may be retained. The minimum temperature value makes it possible to ensure sufficiently strong reaction kinetics. It may be possible to work at room temperature, with an increase in treatment time.

  An alternative is to bring the F and K + ions through, for example, NH5F2 and KC1 solutions, always with acidification of the solution.

  The only surface preparation prior to treatment is to have a clean surface. If the metal resulting from industrial processing is not too fat, a simple thermal degreasing is sufficient; for more safety, it is possible to add, for example, an alkaline pickling with soda or potash.

Examples

  The various treatments were carried out on alloy strips 3916 of the Applicant of composition: Si Fe Cu Alloy Mg Ti 3916 0.19 0.15 0.65 1.35 - 0.08 plated on both sides with a layer of brazing alloy 4045 of thickness representing 10% of the total thickness.

  Treatment 1 (according to patent EP 1067213): Degreasing of the surface with acetone.

  Pickling in a solution of KOH at 10 g / l, at a temperature of 60 ° C. and for 45 s, without neutralization or rinsing.

  Immersion of the sample in a solution of KF at 30 g / l, at a temperature of 60 C and for 30 s.

  Treatment 1-bis: - Same treatment as above but using a solution of KF to g / l.

  Treatment 2 (replacement of KF with NaF): i0 - Degreasing of the surface with acetone.

  Pickling in a solution of KOH at 10 g / l, at a temperature of 60 ° C. and for 45 s, without neutralization or rinsing.

  Immersion of the sample in a solution of NaF at 30 g / l, at a temperature of 60 C and for 30 s.

  2-bis treatment (replacement of KF with NaF): Same treatment as treatment 2, but using a solution of NaF at 50 g / l.

  Treatment 3 (according to the invention): Degreasing of the surface with acetone.

  Stripping in a solution of KOH at 10 g / l, at a temperature of 60 ° C and for 45 s, without neutralization or rinsing.

  Immersion of the sample in a solution of KF at 30 g / l + H2SO4 at 30 g / l, at a temperature of 60 ° C. and for 30 s.

  Treatment 4 (replacement of KF with NaF, with acidification): Degreasing of the surface with acetone Pickling in a solution of KOH at 10 g / l, at a temperature of 60 ° C. and for 45 s, without neutralization or rinsing.

  Immersion of the sample in a solution of NaF at 30 g / l + H2SO4 at 25 g / l, at a temperature of 60 ° C and for 30 s.

  Treatment 5 (according to the invention): Degreasing of the surface with acetone Pickling in a solution of NaOH at 50 g / l at a temperature of 50 ° C. for 20 s. l0

  Neutralization in a solution of HNO3 at 50% by volume, at room temperature and for 15 s.

  Immersion of the sample in a solution of KF at 30 g / l + H2SO4 at 30 g / l, at a temperature of 60 ° C. and for 30 s.

  Treatment 6 (according to the invention): Degreasing with Novaclean 708 product from Henkel Surface Technology for 30 s.

  Stripping in a 50 g / l solution of NaOH at a temperature of 50 ° C. for 20 seconds.

  Neutralization in a solution of HNO3 at 50% by volume, at room temperature and for 15 s.

  Immersion of the sample in a solution of NH5F2 at 15 g / l + KCl at 38 g / l + H2SO4 at 30 g / l, at a temperature of 60 ° C and for 30 s.

  Treatment 6-bis (according to the invention): - Same treatment as before but the final immersion lasts only 15s.

  The test piece described in FIG. 1 was used to evaluate the solderability of the materials having undergone these various treatments. The V consists of a bare strip of alloy 3003, in the H24 state, and of thickness 0.3 mm. A degreasing treatment of 15 minutes at 250 ° C. is applied to the metal to be brazed. No other surface preparation is used and in particular no flux is deposited. Brazing is done in a double-walled glass furnace that allows liquid solder movements and joint formation to be visualized during processing. The thermal cycle is composed of a temperature rise phase up to 600 C with a ramp of approximately 20 C / min, a hold of 2 minutes at 600 C, and a descent at around 30 C / min. Everything is done under continuous nitrogen sweep, with a flow rate of 81 / min.

  The results are qualified by a grade from A to E according to the following scale:

A B C D E

  Joint length 100% 90% 75% 50% 0% formed in relation to the total length The results obtained are as follows: Treatment Brasility 1 E 1-bis E 2 E 2-bis E 3 A 4 E

A 6 A

  6-bis A It is found that the only treatments according to the invention, comprising an acidification of the treatment solution, make it possible to obtain good solderability.

Claims (6)

claims   A method of forming a conversion layer on the surface of an aluminum alloy product to be soldered without flux comprising treating the surface of the product with a solution containing ions K + and F-and one or more acid (s) in an amount such that the pH of the solution is less than 3. to   2. Method according to claim 1, characterized in that the treatment is preceded by an alkaline pickling.   3. Method according to one of claims 1 or 2, characterized in that the acid 15 is sulfuric acid at a concentration of 10 to 40 g / 1.   4. Method according to one of claims 1 to 3, characterized in that the treatment is at a temperature between 30 and 100 C.   5. Method according to one of claims 1 to 4, characterized in that the ions K + and F_ are provided in the form of KF.   6. Method according to one of claims 1 to 4, characterized in that the ions K + and F_ are provided in the form of a KCl + NH5F2 mixture.